This invention relates to electrical storage cells, and, more particularly, to cells producing water during their chemical reactions, such as nickel-hydrogen cells.
Rechargeable cells or batteries are electrochemical devices for storing and retaining an electrical charge and later delivering that charge for useful power. Familiar examples of the rechargeable cell are the lead-acid cell used in automobiles, and the nickel-cadmium cell used in various portable electronic devices. Another type of cell having a greater storage capacity for its weight is the nickel oxide pressurized hydrogen cell, an important type of which is commonly called the nickel-hydrogen cell and is used in spacecraft applications. The weight of the spacecraft storage cell must be minimized while achieving the required performance level, due to the cost of lifting weight to earth orbit and beyond.
The nickel-hydrogen cell includes a series of active plate sets which store a charge electrochemically and later deliver that charge as a useful current, packaged within a pressure vessel that contains the plate sets and the hydrogen gas that is an essential active component of the cell. Each plate set includes a positive electrode, a negative electrode, and a separator between the two electrodes which is impregnated with an electrolyte. In a typical cell, about 40 plate sets are supported on a core under a light compressive loading, with a gas release screen between each plate set and with electrical connector leads extending to each electrode of each plate set. A nickel-hydrogen storage cell delivers current at about 1.3 volts, and a number of the cells are usually connected in series to produce current at the voltage required by the systems of the spacecraft.
A nickel-hydrogen cell used in a satellite is periodically charged by electrical current produced by solar panels on the spacecraft when the satellite is in sunlight, and then later discharged to supply electrical power, when the spacecraft is in shadow or peak electrical power is demanded. A satellite in a low earth orbit may experience up to about 6,000 cycles from light to dark conditions per year, with a corresponding number of cycles of charging and discharging the cells. A typical accepted industry design objective is attaining satisfactory operation through 30,000 cycles of charging and discharging, corresponding to an operating life of 5 years for the satellite in low earth orbit, or more years in other orbits where fewer battery cycles are experienced annually.
The electrolyte used in the cells is typically a 31% solution of potassium hydroxide solute in water, with a freezing point of about -65 C. Under conditions of simulated space operation of cells, freezing of a portion of the electrolyte has been observed at an operating temperature of about -15 C, above the normal freezing point of the electrolyte but within the expected range of spacecraft operations. The performance of the cells is adversely affected when the electrolyte freezes, and freezing cannot be tolerated. One solution to this problem is to limit the operating temperature range of the cells, but this solution would require a substantial increase in weight, because the temperature range is essentially governed by the space environment. Special insulation or heaters would be required to ensure the maintenance of an acceptable temperature, and these modifications would add weight and possibly consume energy, both of which are disadvantages.
There therefore exists a need for an approach that minimizes problems arising from the freezing of the electrolyte during electrical discharge of a storage cell of the nickel-hydrogen type, under ambient temperature conditions a few degrees above the nominal freezing point of the electrolyte. The present invention fulfills this need, and further provides related advantages.